The invention relates to a controllable antenna structure comprising a set of antennas and means for controlling the set of antennas, said control means including at least one phase shifter and computing means for determining a phase shift of an electromagnetic current transmitted to or received by an antenna of the set of antennas, said phase shift being applied to said antenna to achieve a requested radiation pattern.
The present invention further relates to a radio station for use in a communication system, said radio station having such a controllable antenna structure, to a flexible support carrying such a controllable antenna structure, and to a communication system having at least one primary radio station and at least one secondary radio station, said secondary radio station having a controllable antenna structure. Said communication system may be, for example, a wireless communication system that will operate at high frequencies. Said communication system can be a terrestrial and/or a satellite cellular mobile radio system or any other suitable system.
A controllable antenna structure of the above kind is known from the handbook xe2x80x98Mobile Antenna systems Handbookxe2x80x99, K. Fujimoto et al., Artech House, Inc., 1994, pp. 448 to 451.
The known controllable antenna structure is a phased-array antenna system wherein a directive radiation pattern is obtained through the use of several antenna elements. The known controllable antenna structure is implemented in a land mobile satellite communication system. The set of antennas is fixed on the roof of a vehicle and is in communication with a primary radio station, which is a satellite.
The known set of antennas is in relation with means for controlling the radiation pattern of said set of antennas. An initial acquisition is first realized by a full azimuth search for the strongest received signal and then said control means, which include phase shifters, control the phase shift of an electromagnetic current transmitted to or received by an antenna in order to track the direction of the strongest received signal.
When several antenna elements radiate or receive with appropriate phase shifts, it is possible to achieve a desirable radiation pattern constituted of lobes in the desired directions for a phased-array antenna. The use of a phase shifting principle to achieve a requested radiation pattern of a set of antennas requires that the antennas be separated by a distance of the order of a wavelength. As an example, in third generation mobile phones the wavelength is 30 to 15 cm for a system operating in the range of 2 GHz or 1 GHz. In the above described controllable antenna structure, antennas are fixed to a large rigid structure.
A major drawback of the controllable antenna structure in accordance with the prior art is that for these mobile phones, dimensions of a rigid structure to which antennas are fixed, need to be larger than the size of the mobile equipment itself. Moreover it has to be noted that a set of antennas using the phase shifting principle needs to have a consequent number of antennas to be profitable. The less important the antenna number is, the less directive the resulting radiation pattern can be. Said radiation pattern is defined by the summation of the inherent radiation patterns of each antenna and is modified by relative phase shift changes.
The present invention takes the following aspects into consideration. A mobile user presents a large enough surface to arrange a high number of antennas. Furthermore, the invention takes into account that this surface is not necessarily a rigid surface.
It is an object of the invention to provide a controllable antenna structure as described in the first paragraph, which can be arranged on a flexible surface.
To this end, the controllable antenna structure according to the invention is characterized in that the set of antennas is fixed to a flexible support, and the controllable antenna structure further comprises means for detecting motion of the flexible support to which antennas are fixed, the computing means determining the phase shifts as a function of said motion.
The strong directivity allowed by the use of the phase shifting principle is consequently available for mobile equipment. This allows to reduce interference and to lower the power required for a communication.
Nevertheless, interference can still exist when only phase shifting is used to achieve a requested radiation pattern.
It is another object of the invention to provide a controllable antenna structure which can be steered selectively towards a given direction without being affected by parasitic information coming from other directions.
To this end, the controllable antenna structure according to the invention is characterized in that the control means further include at least one variable gain multiplier for multiplying an amplitude of the electromagnetic current transmitted or received by an antenna in order to achieve a requested radiation pattern, the computing means determining the gain as a function of said motion.
More generally, the present invention comes within the scope of mobile radio stations in a communication system that needs directional antennas. The use of directional antenna allows to increase the traffic capacity substantially, to improve the signal quality, but also to reduce electromagnetic radiation on the human body. Consequently, the present invention is also a contribution to the ensurance of providing a better service quality to users.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.